Production of hard surfaces on base metals



PRODUCTION OF HARD SURFACES ON BASE METALS Filed Apr. 29, 1954, Ser. No. 426,553

1 Claim. (Cl. 204-37) No Drawing.

This invention relates to the formation of hard metallic surfaces or layers on base metals.

A wide variety of hard facing materials on base metals are used today for industrial purposes. These hard facing materials are generally provided on the base metal either by welding the hard facing material to the base metal or by treating the base metal in such fashion as to harden its surface.

We have now discovered that hardened surfaces composed of titanium and certain other transition metals may be provided on a base metal by a Wholly different procedure. This procedure involves forming a cladding layer of the transition metal on the base metal and either simultaneously therewith or thereafter causing the transfer of a transition metal-hardening element from the mass :of the base metal to the cladding layer of the transition metal. Our resulting novel method of forming on a base metal a layer of hard intermetallic compound of a transition metal such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten comprises forming a fuse salt bath composed essentially of a halide of the transition element and a diluent halide salt. A cathode composed essentially of the base metal, and further containing a significant amount of an interrnetallic compound-forming element such as carbon, nitrogen, boron or silicon, is then immersed in the fused salt bath as the cathode of an electrolytic cell. The bath is electrolyzed between this cathode and an anode with the resulting deposition of a layer of the transition metal on the cathode. Either simultaneously with this electrodeposition of the transition metal on the cathode or subsequent thereto, the cathode bearing the deposited layer of the transition metal is heated sufficiently to effect thermal diffusion of the intermetallic compound-forming element from the base metal into the transition metal layer.

The base metals on which a hardened layer of a transition metal may be formed by the practice of our invention include all metals capable of holding carbon, nitrogen, boron or silicon either interstitially or in solid solution. iron and steels are representative of such base metals, although other metals and alloys such as copper, brass, bronze, nickel, or the transition metals and their alloys, are capable of being provided With a hard surface by the method of our invention.

The base metals which are used in the practice of our invention must contain a significant amount of at least one of the intermetallic compound-forming elements carbon, nitrogen, boron and silicon. The intermetallic compound-forming element may be present in the base metal either as a component uniformly distributed throughout the entire body of the base metal, or it may be present in large part or even exclusively in a relatively thin surface layer of the base metal. For this reason, the amount of the intermetallic compound-forming element present in the base metal cannot be stated categorically. Thus, it can only be stated that, in the case of steel, for example,

nited States Patent ice from 0.2% to 1.2% carbon, from a few hundredths percent to 0.2% nitrogen, from about 0.01% to 0.05% boron and from a few hundredths percent up to about 2% silicon in various steels will provide a suflicient amount of the intermetallic compound-forming element for thermaldiffusion into the transition metal layer on the base metal within a reasonable period of time. In general, it can be said that the upper limit of the amount of such inte'r metallic compound-forming element which may be pres-' compound-forming element, as in the case of carburized' or nitrided steels, the amount of the intermetallic com pound-forming element present in this layer is wholly adequate for forming by thermal diffusion a hard intermetallic compound of the transition metal deposited on' the surface of the base metal.

The transition metals which are useful in the practice of our invention include titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. Each of these metals is capable of being electrolytically deposited on the surface of the aforementioned base metals, and each of these transition metals further forms hard intermetallic compounds with carbon, nitrogen, boron or silicon. When the base metal is composed of one of the transition metals or its alloys, this transition metal base body may be provided with a useful content of carbon, nitrogen, boron or silicon by any con-' ventional technique prior to the formation thereon of a finish surface of the transition metal-intermetallic compound pursuant to the method of our invention. 1

All of the aforementioned transition metals are capable of being electrolytically deposited von the aforementioned base metals in a fused salt bath containing carrier ions of the transition metal. The electrodeposition of titanium metal, which is representative of the other aforemen tioned transition metals, may be effected for example by the relatively high temperature cladding technique de scribed in the copending application of Sibert and Bur well, Serial No. 383,401, filed September 30, 1953, and now Patent No. 2,828,251. Pursuant to this technique; the titanium cladding layer is deposited on the cathode by electrolyzing between the cathode and a transition metal carbide-containing anode an anhydrous fused halide salt bath containing at least 5% by weight of a halide of the transition metal. By maintaining a cathode current density of about 25 to 200 amperes per square decimeter and by maintaining the cell temperature between 900 and 1000 C., the resulting electrodeposited transition metal adheres firmly to the base metal cathode in the form of a cladding layer. The cladding layer of the transition metal may also be formed by the procedure of the copending application of Bertram C. Raynes, Serial No. 383,402, filed September 30, 1953, which issued March 26, 1957, as United States Patent 2,786,809. The Raynes cladding method is characterized by the incorporation in a fused halide salt bath containing a significant amount of an alkali metal-transition metal double fluoride of a significant amount of oxygen advantageously added in the form of water. When this bath is electrolyzed at a temperature above its melting point between a solid anode and a cathode composed of the desired base metal, the base metal cathode is clad by a metal-to-metal bond with the transition metal.

It must be understood, however, that our invention is not limited to the use of either of the two aforementioned procedures for electrolytically forming the cladding layer of the transition metal on the base metal. Our invention may be practiced by the use of any' other procedure whichwill electrolytically form on the surfaceof the base metal a relatively uniform and adherent" layer of one of the aforementioned transition metals. Regardless of the method which is used for forming the deposit of transition metal on the base metal, the thermal difiusion of the intermetallic compound-forming element from the base metal to the transition metalpromotes the forma tion of a strong'physical bond between the base metal and the resulting hard facing thereon. V

The thermal diffusion of the intermetallic compoundforming element'from the base metalto the layer of transition metal deposited thereon may take place either simultaneously with or subsequent to. the deposition of the transition metalon the base metal.- Thus, if the deposition of the transition metal on the base metal takes place at -a temperature sufiicientlyhigh to promote thermal diffusion of the intermetallic compound-forming element from the'base metal to thedeposited transition metal, then the thermal diffusion will take place sitnultaneously with the electrodeposition of the transition metal. In general, it'canbe stated that electrolytic operations carried out within the temperature range of 800 to 1000 C. (and higher if volatilization of the bath components permit) will effect the desired diffusion of the inter-metallic compound-forming element from the base metal to'the transition metal simultaneously with the electrodeposition of the latter, and it .will be seen that this result is generally obtained by the practice of the high temperature-low voltage procedure described and claimed in the aforementioned Sibel-t and Bu'rwell application; On the other hand, the process described in the aforementioned Raynes application, which is carreid out at higher voltages and generally in the range of to 8 volts, can be practiced not :only atthese same elevated temperatures but alsoat considerably lower temperatures of the order of 500 to 600 C. When the'transition metal is electrodeposited on the base metal :at such low temperatures that there is no significant diffusion of the inter-metallic compound-forming element from the base metal to the deposited transition metal, we have found that this diffusion can be effected either by raising the temperature of the bath while the transition metal-bearing base metal cathode is still immersed therein or by subsequently heatingzthe transition metal-bearing base metal in a subsequent operation either before -.or.after the entrained electrolytic bath salts have been-removed from the transition metal deposit. Ofcou-rse, such subsequent heating of the transition metal-coated base metal should be carn'edout in an inert atmosphere, such as an argon atmosphere or a vacuum, in order to prevent unwanted deterioration ormodification'of the transition metal layer.

The thickness of'the resulting hard intermetallic compound of the transition metal on the base metal canvary extensively and is determined primarily upon the use to which the resulting product is to be put. In general, however, We have found it advantageous to form layers ranging from a few thousandths 'of an inch up to about one sixty-fourth of an inchof the transitionmetal, and the resulting hard layercomposed predominantly of the intermetal-lic -compound of the'transition metal'and carbon, nitrogen, boron or silicon is sufiiciently substantial and coherent to meet most commercial requirements for hard-faced materials. The hardness of theseintermetallic compounds of the transition metals formed by thepractice of our invention ranges from about 400 to about 1000 Vickers hardness number.

4 The practice of our invention can be further illustrated by the following examples:

Example I 5 Three hundred forty parts of the potassium double fluoride of titanium plus 2% by weight of water were mixed with 1800 parts 'by weight of sodium chloride. This mixture was then added to a heated graphite crucible maintained at a temperature of 840-900 C. in 10 an atmosphere of argon. When fusion of the salt mixture occurred, a cathode shape composed of high carbon steel (containing 0.8% C)' was immersed in the fused salt bath. A direct current voltage of about 6 volts was applied between the base metal cathode and the crucible which served as the anode. Cathode-current densities of about 200-300 amperes per square decimeter were maintained and the electrolysis was continued for about /2 hour. The cathode was then removed from the bath and was cooled in argon to room temperature. The resulting titanium-clad high carbon steel piece was finally leached free of residual salts, and the titanium carbidecontaining layer on the steel was found to be about & inch thick having a Vickers hardness number of about 500. 25

Example II The fused bath described in Example I, but without the addition of water, was. electrolyzed at a voltage of about 3.4 volts and at a temperature of about 860 C. for a period of 10 minutes using a cathode of Ketos steel (containing 0.9% carbon). Theresulting clad layer of hardened titanium was 0.002 inch in thickness and had a Viclcers hardness of about 700.

Example III p The bath described in Example II was electrolyzed at 3.5 volts and at a temperature of 1000-1050 C. In a period of 20 minutes a cathode shape fabricated of 1.2% carbon tool steel received a clad layer 0.003 inch in thick- 40 ness and composed of carbon-hardened titanium having a Vickers hardness of about 900.

We claim:

A method of providing a coating of titanium carbide several thousandths ,of an inch in thickness on a steel substrate which comprises: providing a fused salt bath consisting essentially of at least one titanium halide and at least one diluent halide salt from the group consisting of alkali metal halides and alkaline earth metal halides; providing an anode and at least one steel cathode in electrical contact with said bath, said cathode containing at least about 0.8% by Weight of carbon; passing an electrolyzing current through said fused salt bath and between said anode and cathode, for a time suflicient to deposit at least a few .thousandths of an inch of titanium on said steel substrate and maintaining the base and coating at a temperature of at least about 840 C. for a time sufficient to form .a coating of titanium carbide on said steel base by thermal diffusion of the carbon into the coating.

References Cited in the file of this patent OTHER REFERENCES 7 Wright Air Development Center Technical Report 53-317; Electrodeposition of Titanium and Zirconium, December 1953.

Websters New International Dictionary, second edition, p. 2466. V 

